Katherine Akulov

Comprehensive study of ultrafast excited-state proton transfer in water and D2O providing the missing RO(-)···H(+) ion-pair fingerprint.

Steady-state and time-resolved optical techniques were employed to study the photoprotolytic mechanism of a general photoacid. Previously, a general scheme was suggested that includes an intermediate product that, up until now, had not been clearly observed experimentally. For our study, we used quinone cyanine 7 (QCy7) and QCy9, the strongest photoacids synthesized so far, to look for the missing intermediate product of an excited-state proton transfer to the solvent. Low-temperature steady-state emission spectra of both QCy7 and QCy9 clearly show an emission band at T < 165 K in H2O ice that could be assigned to ion-pair RO(-)*···H3O(+), the missing intermediate. Room-temperature femtosecond pump-probe spectroscopy transient spectra at short times (t < 4 ps) also shows the existence of transient absorption and emission bands that we assigned to the RO(-)*···H3O(+) ion pair. The intermediate dissociates on a time scale of 1 ps and about 1.5 ps in H2O and D2O samples, respectively.

Acid effect on photobase properties of curcumin.

Femtosecond UV-vis pump-probe spectroscopy was employed to study the acid effect on curcumin in the excited state. Curcumin in solutions of weak acids was found to be a photobase forming a protonated curcumin within a few tens of picoseconds from the time of excitation. The excited-state protonation reaction is also observed in the steady-state emission spectrum as a new red emission band with a maximum at 620 nm in the presence of weak acids. The transient pump-probe spectrum consists of four spectral bands, two emission bands, and two absorption bands. We assign a transient absorption band at ∼600 nm and an emission band at ∼540 nm to the neutral ROH form of curcumin. An absorption band at ∼500 nm and an emission band at 620 nm are assigned to the protonated ROH2(+) form of curcumin.

Acid effect on excited Auramine-O molecular rotor relaxations in solution and adsorbed on insulin fibrils

Steady-state and time-resolved UV-Vis spectroscopy techniques were employed to study the non-radiative process of Auramine-O (AuO). We focused our attention on the ultrafast nonradiative decay of Auramine-O in water and on the acid effect on Auramine-O spectroscopy. We found that weak acids like formic acid shorten the excited-state decay times of both the emission and the transient pump-probe spectra of Auramine-O. We found three time domains in the relaxation of the excited states back to the ground state. In mixtures of acetic and formic acids, the three decay times associated with the relaxation process are shorter in the presence of formic acid in Auramine-O solutions. We qualitatively explain the very large non-radiative rate in water and in formic-acetic acid mixtures by a protic nonradiative model proposed by Sobolewski and Domcke. The steady-state emission spectrum of AuO adsorbed on insulin fibrils consists of two bands assigned to protonated and deprotonated forms and the emission intensity increases by three orders of magnitude. We conclude that the nonradiative process prevails in the liquid state, whereas when AuO is adsorbed on fibrils the nonradiative rate is reduced by three orders of magnitude and thus enables a slow ESPT process to occur.

Temporal Dynamics of Localized Exciton-Polaritons in Composite Organic-Plasmonic Metasurfaces

We use femtosecond transient absorption spectroscopy to study the temporal dynamics of strongly coupled exciton-plasmon polaritons in metasurfaces of aluminum nanoantennas coated with J-aggregate molecules. Compared with the thermal nonlinearities of aluminum nanoantennas, the exciton-plasmon hybridization introduces strong ultrafast nonlinearities in the composite metasurfaces. Within femtoseconds after the pump excitation, the plasmonic resonance is broadened and shifted, showcasing its high sensitivity to excited-state modification of the molecular surroundings. In addition, we observe temporal oscillations due to the deep subangstrom acoustic breathing modes of the nanoantennas in both bare and hybrid metasurfaces. Finally, unlike the dynamics of hybrid states in optical microcavities, here, ground-state bleaching is observed with a significantly longer relaxation time at the upper polariton band.

Temporal dynamics of strongly coupled exciton-localized surface plasmons beyond Rabi oscillations

Strong coupling between excitons and light leads to the formation of hybrid states with mixed properties of light and matter. As a result, interesting physical phenomena have been observed at room temperature, e.g. Bose–Einstein condensation and superfluidity, and novel applications are emerging, such as low threshold lasers and quantum devices. Recently it was shown that metasurfaces of aluminum nanoantennas coated with molecular J-aggregates can provide an excellent platform for the formation of strongly coupled exciton-localized surface plasmons (X-LSPs). However, their optical nonlinearities and temporal dynamics are still not well understood. In this work, we use femtosecond pump-probe spectroscopy to study X-LSPs in such composite Al/molecular metasurfaces on time scales that are longer than their Rabi oscillation period. We study the linear and nonlinear optical properties of the uncoupled and hybrid systems and find that the nanoscale plasmonic confinement in metallic nanoparticle cavities introduces intriguing new ultrafast phenomena in the strong coupling regime. These include modifications of the hybrid system due to femtosecond changes in the molecular environment, picosecond oscillations due to acoustic breathing modes of the nanoantennas, and long relaxation times of the nonlinear perturbation at the upper X-LSPs frequency band.

Picosecond optomechanical oscillations in metal-polymer microcavities

We experimentally study mechanical vibrations in planar Fabry-Perot microcavities made of metallic mirrors and a polymer spacer, using broadband pump-probe spectroscopy. These acoustic waves oscillate at a picosecond time-scale and result in spectral oscillations of the cavity transmission spectrum. We find that the oscillations are initiated at the metal mirrors and that their temporal dynamics match the elastic modes of the polymer layer, indicating that mechanical momentum is transferred within the structure. Such structures combine the strong optical absorption of metals with the elasticity and the processability of polymers, which open the road to a new class of optomechanical devices.